1. Technical Field
The present invention relates to a Radio Frequency Identification (RFID) tag circuitry and antenna used in combination with a Programmable Logic Device (PLD).
2. Related Art
Radio Frequency Identification (RFID) is a method of remotely storing and retrieving data and identification using devices called RFID tags (or RFID transponders). RFID tags contain antennas to enable them to receive and respond to radio-frequency queries from an RFID transceiver. An RFID tag can be attached to or incorporated into a product, such as by placing the RFID tag in an adhesive sticker attached to the product.
FIG. 1 shows an RFID reader antenna 2 and a tag antenna 4, illustrating use of a loop antenna used to form both the RFID tag and the RFID reader. As illustrated, the antennas 2 and 4 operate similar to a transformer to magnetically couple the antennas when one of the loops, in the case of the reader antenna 2, is energized with an alternating current to creating a magnetic field shown by dashed lines 3. The tag loop antenna 4 acts like the secondary of a transformer by extracting energy from he magnetic field 3 created. The tag antenna 4 is connected to a tuning capacitor 5 and circuitry 6 for communicating an identification signal to the reader. Although shown as loop antennas, the antennas 2 and 4 can take other forms as long as the magnetic coupling 3 occurs.
FIG. 2 shows an equivalent circuit for the RFID tag along with the RFID reader antenna. The RFID reader antenna creates an inductance 12 that is coupled with the RFID tag antenna. The tag antenna also produces a resistance 8. The tag antenna inductance 14 is connected in parallel with a tuning capacitor 5. The tag antenna inductance 14 and tuning capacitor 5 are then further connected in parallel with RFID tag communication circuitry 6. The tuning capacitor 5 is sometimes integrated with the tag circuitry 6.
RFID tags were introduced in the 1940s, but small size components were needed to make them effective, preventing any significant use until the 1980s. Even smaller components for the RFID tags since the 1980s have recently increased the use of RFID tags.
RFID tags can be either active or passive. Passive RFID tags do not have their own power supply. The minute electrical current induced in the tag antenna from the incoming radio-frequency scan provides enough power for the tag to send a response. Due to power and cost concerns, the response of a passive RFID tag is necessarily brief: typically just an ID number. Lack of its own power supply makes the device quite small. As of 2004, the smallest passive devices commercially available measured 0.4 mm×0.4 mm, and are thinner than a sheet of paper. Passive tags have practical read ranges that vary from about 10 mm up to about 6 meters.
Active RFID tags, on the other hand, must have a power source, and may have longer ranges and larger memories than passive tags, as well as the ability to store additional information sent by the transceiver. In 2004, the smallest active tags are about the size of a coin and have a range greater than 10 meters.
As passive tags are much cheaper to manufacture and do not depend on a battery, the vast majority of RFID tags in existence are of the passive variety. As of 2004 tags cost from US $0.40. The aim is to produce tags for less than US $0.05.
As of 2004, there are four different kinds of tags commonly in use. They are categorized by their radio frequency: low frequency tags (between 125 to 134 kilohertz), high frequency tags (13.56 megahertz), UHF tags (868 to 956 megahertz), and microwave tags (2.45 gigahertz).
Low-frequency RFID tags are commonly used for animal identification, and automobile key-and-lock systems, and electronic toll collection. With a paper thin size RFID tag, animal or pet identification is provided by embedding small chips in the pet so that they may be returned to their owners if lost. Electronic toll collection using the RFID tags is provided by California's FasTrak and Illinois' I-Pass system. The information is used to debit the toll from a prepaid account. The system helps to speed traffic through toll plazas.
High-frequency RFID tags are used in library book tracking, building and employee access control, and apparel item tracking. These badges need only be held within a certain distance of the reader for authentication. Even higher frequency UHF RFID tags are commonly used commercially in cargo container and truck trailer tracking in shipping yards. Microwave RFID tags are used in long range access control for vehicles.
A PLD, such as a Complex Programmable Logic Device (CPLD) or a Field Programmable Gate Array (FPGA), typically provides significantly more circuitry than a conventional circuit connected to an antenna to create an RFID tag. A PLD, however, provides a programmable circuit that can be configured to adapt to various user needs. The complexity and size of the PLD has in the past prevented its use with an RFID tag.
For reference, a block diagram of conventional components of one type of PLD, an FPGA, is shown in FIG. 3. The FPGA includes input/output (IOBs) blocks 22 (each labeled 10) located around the perimeter of the FPGA, multi-gigabit transceivers (MGT) 24 interspersed with the I/O blocks 22, configurable logic blocks 26 (each labeled CLB) arranged in an array, block random access memory 28 (each labeled BRAM) interspersed with the CLBs, configuration logic 32, configuration interface 34, on-chip processor 36 and an internal configuration access port (ICAP) 30. The FPGA also includes other elements, such as a programmable interconnect structure which interconnects the various programmable elements (e.g., MGT, CLB, BRAM, IOB, and the like) and a configuration memory array, which are not illustrated in FIG. 3. Although FIG. 3 shows a relatively small number of I/O blocks 22, CLBs 26 and block RAMs 28 for illustration purposes, it is understood that an FPGA typically includes many more of these elements.
In general, the FPGA of FIG. 3 is programmed in response to a set of configuration data values that are loaded into a configuration memory array of the FPGA. The internal configuration memory cells define how the CLBs 26, IOBs 22, BRAMs 28 and interconnect structure are configured. Configuration data is provided to the configuration memory cells as a bitstream from an external memory (e.g., an external PROM or PC) via configuration interface 34 and configuration logic 32. The configuration interface 34 can be, for example, a parallel select map interface, a JTAG interface, or a master-serial interface. As another alternative, the FPGA can be reconfigured by rewriting data in the configuration memory array using the ICAP 30.
With user needs for a configurable or programmable circuit, such as a PLD, increasing, as well as the needs for RFID circuitry increasing, a way of combining features of a PLD with an RFID tag is desirable.